Method and apparatus for fluorescent x-ray analysis

ABSTRACT

1. IN A BALANCED FLUORESCENT RADIATION DETECTION APPARATUS UTILIZING A PRIMARY RADIATION SOURCE, A SINGLE PHOTODETECTOR HAVING A PHOTOSENSITIVE FACE, SEPARATE SCINTILLATOR MEANS IN OPTICAL COMUNICATION WITH SAID PHOTOSENSITIVE FACE AND SHIELDED FROM SAID PRMIARY RADIATION SOURCE AND OPTICALLY SHIELDED FROM EACH OTHER, FIRST AND SECOND RADIATION FILTER MEANS WHEREIN SAID FIRST FILTER MEANS, IN CONTRAST TO SAID SECOND FILTER MEANS, SELECTIVELY ATTENUATES FLUORESCENT RADIATION IN AN NORROW ENERGY BAND CORRESPONDING TO THE FLUORESCENT RADIATION ENERGY OF AN ELEMENT OF INTEREST, THE IMPROVEMENT WHEREIN SAID FIRST AND SECOND FILTER MEANS ARE CONCURRENTLY POSITIONED FOR FLUORESCENT RADIATION DETECTION IN ASSOCIATION WITH SEPARATE ONES OF THE SCINTILLATOR MEANS, AND A LIGHT FILTER FOR UNIFORMLY ATTENUATING LIGHT ENERGY BY AT LEAST 10 PERCENT IS INTERPOSED BETWEEN THE SCINTILLATOR MEANS ASSOCIATED WITH SAID FIRST FILTER MEANS AND THE PHOTOSENSITIVE FACE OF SAID PHOTODETECTOR.

United States Patent [191 Ashe et al.

[ METHOD AND APPARATUS FOR FLUORESCENT X-RAY ANALYSIS [75] Inventors:John B. Ashe; Peter F. Berry; James D. Hall, all of Austin, Tex.

[73] Assignee: Texas Nuclear Corporation, Austin,

Tex.

[22] Filed: June 4, 1973 [21] Appl. No.: 366,488

[52] US. Cl 250/273, 250/272, 250/274 [51] Int. Cl. G01n 23/22 [58]Field of Search 250/272, 273, 274, 277,

[56] References Cited UNITED STATES PATENTS 3,467,824 9/1969 Boyce etal. 250/273 Jan. 7, 1975 Brinkerhoff et al. 250/277 Dykeman 250/274Primary Examiner-William F. Lindquist Attorney, Agent, or FirmCharles H.Thomas, Jr.; Walter C. Ramm; Peter J. Sgarbossa [57] ABSTRACT Aradiation analyzer which generates X-ray fluores- 7 Claims, 6 DrawingFigures I SINGLE DISPLAY SCALER GATE I CHANNEL 9s so as ANALYZER 2| I7 12 5 l l9 SINGLE PHOTOMULTIPLIER SCALER FACTOR GATE CHANNEL 12 E e9 92 a7ANALYZER 20 .8 GATED I l5 osc |4| 94 CONTRO l l i FROM 63 Patented Jan.7, 1975 V 3,859,525

4 Sheets-Sheet 2 Patented Jan. 7, 1975 3,859,525

4 Sheets-Sheet 5 COUNTS -scA 24 I --SCA 25 I I 27 I PULSE HEIGHT FIG. 4

\ I? I8 H8 PHOTOMULTIPLIER SINGLE CHANNEL ANALYZER SINGLE CHANNEL AMPANALYZER 23 SINGLE CHANNEL ANALYZER FIG. 6

Patented Jan. 7, 1975 4 Sheets-Sheet 4 TnonANn APPAriATUs FORFLUORESCENT X-RAfY ANALYSIS B cxoRouNDor THE INVENTION In manyexistingjnon-dispersive fluorescent x-ray analyzers, sequentialcomparative radiation measurements are necessary, per haps with balancedfilters to achieve adequate energy resolution. In the comparativemeasurement technique, substances of known and unknown compositions areirradiated. Fluorescent x-ray radiation emanating'there' from ismeasured in sequential operations of the radiation analyzer, so that themeasurement. results ;of the substance of unknown composition can bedetermined withrespect to the measurement results of the substance ofknowncomposi-v tion. Howevenvariatidhsiin measuring technique,instrument gain, and voltage fluctuations significantly affect theresults obtained; v

Balanced filters together transmit a narrow band of x-ray energies suchthat transmission by each filter of a filter 'pair outside of this bandis balanced, but transmission within'the band is significantlydifferent. The difference in the signal transmitted through the-filtersis proportional to the incident radiation intensity in the narrow bandof unbalance. This narrow band of unbalance can bechosen to include thecharacteristic fluorescent radiation energy ;for an element of interest,and is typically less than S'KeV in width. The bal In other types ofbalanced filter analyzers, dual radiation detectors or dissimilarscintillators are utilized, one with the transmitting filter and theother with the absorbing filter. The counts from the two detectors arecompared to determine the amount of radiation attributable to theelements of interest. In the known dissimilar scintillator system therespective signals are separated by rise time techniques. Thisdifferential count is indicative of the quantity of the element which ispresent, as previously discussed.

A treatment of balanced filters generally may be found in the book X-RayAnalysis Papers, edited by William Parish, Centrex Publishing Company,Eindoven, 1965, at pages 36 and 37.

SUMMARY OF THE INVENTION It is an object of the present invention toprovide fluorescent radiation detection apparatus utilizing a singlephotodetector to obtain a measurement proportional to the quantity of anelement of interest which is present. Operation of the apparatus isperformed in a single measurement cycle.

It is a further object of the present invention to achieve measurementin a single cycle, or counting urements to be made with substantiallyhigher energy resolution than the inherent resolution of the detector.

step, using a single photodetector in order to eliminate the possibilityof error due to movement of the detector or specimen between countingperiods or due to internal electrical variations within the instrumentor due to unpredicted fluctuations in background radiation dur-.photodetector gain and obviates the introduction of errorcaused bydiffering photodetector response characteristics. The present inventionalso reduces the expense required to manufacture a radiation detectionThe energy resolution is governed by the pass band of the filter pairrather than the inherent properties of the detector. i i

Stated another way, the v used herein, refers to a pair of filtershaving different transmission properties'i'na narrow'energy-band ofininvention can be used to replace a conventional detecterest but havingtransmission properties closely matched for all other energies ofinterest. The quantitaterm balancedfilters, as

apparatus by obviatingthe necessity for a second photodetector,

It is a further object of this invention to provide a system. forsimultaneously quantifying radiation signals tive determination of theamountof an element of interest is determined by the difference of thetransmitted signals passed by the .filters. One filter, hereinaftertermed the transmitting filter, passes a greater portion of theradiation in a narrow energy range of interest than does the otherfilter, hereinafter termed the 5 237 2 ffl d n Man 24 1972 absorbingfilter. I

Several types of balanced filter analyzers aretcurrently in use. Onetype sequentiallyinterposes each of a pair of filters between thesubstance to be, analyzed and a scintillator during a measuring cycle.'Aradiation count is taken for a predetermined time with one of the Ifilters interposed between the detector and the speciradiation countwith the absorbing filter is indicative of the extent to which theelement .of interest is present.

'frorn'a plurality of independent scintillators with a singlephotodetector.

In addition the fadiation detection apparatus of this tor system used toconcurrently count radioactive events in a plurality of samples. Forexample, the radiation detection apparatus of this invention, using asingle "photodetector, can be used to replace the crystals and In abroad aspect this invention is, in a radiation detection apparatusutilizing a plurality of scintillators, the improvement comprising asingle photodetector for generating electrical pulses in opticalcommunication with the scintillators, a light filter for attenuatingincident light energy by at least 10 percent positioned between thephotodetector and at least one and less than all of said scintillators,a first signal analyzer connected v diation count with the transmittingfilter exceeds the to said photodetector for accepting electrical pulsesin a predetermined amplitude range, and a second signal analyzerconnected tosaidphotodetector and set to accept electrical pulses havingamplitudes falling within the aforesaid predetennined energy range butdiminished by the amount of which the aforesaid light filter attenuatesincident light energy.

While the terms filter.and. scintillator" may be used in the singular,they should also be taken to include those situations where one filteror scintillator may be comprised of several identical componentelements.

In one embodiment of the invention, a pair of balanced radiation filtersare employed in addition to a light filter. Each filter in thebalancedfilter pair absorbs radiation to the same extent outside of anarrow radiation energy range of interest. An imbalance between thefilters of a balanced filter pair implies differing absorption in thenarrow energy range of interest. The difference in the signals passed bythe filters in the balanced filter pair is proportional to the imbalanceof the radiation intensity in the narrow energy range of interest.

While attenuation of at least 10 percent has been specified as beingdesirable, the minimum acceptable degree of attenuation may be more oreven less, depending upon the response function of the radiationdetectors involved. The most important factor is that there must besignificant resolution between the pulse spectra generated from each ofthe scintillation detectors.

BRIEF DESCRIPTION OF THE INVENTION The invention may be more fullyexplained and completely understood by reference to the accompanyingdrawings in which:

FIG. 1 is a schematic representation of one embodiment of thisinvention,

FIG. 2 is an enlarged cross sectional elevational view of the radiationprocessing assembly taken along the lines 22 of FIG. 3,

FIG. 3 is an enlarged plan view of a radiation processing assembly takenalong the lines 33 of FIG. 2,

FIG. 4 graphically depicts the pulse height spectra recorded using theapparatus of this invention,

FIG. 5 illustrates in partial section a portion of the apparatus of oneembodiment of this invention, and

FIG. 6 depicts schematically a portion of an alternative embodiment ofthis invention.

- DETAILED DESCRIPTION or THE INvENTIoN Referring now to FIG. 1, thereis illustrated a fluorescent radiation detection apparatus 10 employinga primary radiation source 11'. A single photomultiplier orphotodetector 12 is utilized having a photosensitive surface 15.Detecting elements in the form of distinct scintillator means 17 and 18are optically shielded from each other and are in optical communicationwith the photosensitive face 15. A shield 16' is provided to shielddetecting elements 17 and 18 from primary radiation source 11'. Oneradiation filter 19 of a balanced filter pair 'is located proximate tothe scintillator 17. Radiation absorbing filter l9'strongly absorbsfluorescent radiation of the element of interest. Radiation transmittingfilter 20 is located proximate to the scintillator 18. Filter 20 hasenergy response attenuation characteristics similar to those of filter19 at energy levels other than in a predetermined narrow energy rangewhich is characteristic of the element of interest. Filter 20 onlyweakly absorbs radiation in this predetermined narrow energy range.While the scintillators l7 and 18 are both in optical communication withphotosensitive face 15, a light filter 21 for uniformly attenuatingincident light energy typically by at least 10 percent is positionedbetween the photosensitive surface 15 and the scintillator 17.Alternatively, the light filter 21 could be interposed between thephotosensitive surface 15 and scintillator 18. It is only imperativethat this light filter be used with either the scintillator 17associated with filter 19 or the scintillator 18 associated with filter20, but not with both.

The photomultiplier 12 is connected to an amplifier 23 and then inparallel to separate single channel analyzers 24 and 25. The electricalpulse energy window of the single channel analyzer 24 is indicated bySCA 24 in FIG. 4. Similarly, the electrical pulse energy window ofsingle channel analyzer 25 is indicated as SCA 25 in FIG. 4. Byreference to FIG. 4 it can be seen that the energy window of analyzer 24is significantly less than the energy window of analyzer 25, and thereis substantial resolution between the pluse spectra 28 and 27 from thescintillators 17 and 18 respectively. Since the light filter 21 isinterposed between the photosensitive surface 15 and the scintillator17, the spectrum 28 of scintillations from the scintillator 17 is at amuch lower level than the spectrum of scintillations from scintillator18. The average pulse energy under curve 28 is about 30 percent of theaverage pulse energy under curve 27 if filter 21 attenuatesscintillations from scintillator 17 by about 70 percent, as in theillustrated embodiment. The area under the curve 28 would be identicalto the area under the curve 27 if the balanced radiation filters l9 and20 were not present. Since the filters l9 and 20 are positioned asindicated, however, there is a difference in the two areas. Thisdifference is proportional to the amount of the fluorescent radiationexcited by source 11' in the element of interest. The

count differential between the spectrum 27 and the spectrum 28 istherefore proportional to the amount of an element of interest which ispresent in the specimen under study.

The outputs of single channel analyzers 24 and 25 are connected to acounting means indicated generally as 26. In the embodiment of FIG. 1,the outputs of single channel analyzers 24 and 25 are gated respectivelyby gates 87 and 88 into separate scalers 89 and 90 for a preselectedtime interval as determined by the program control unit 91. The outputfrom single channel analyzer 24 passes through a factor gate 92 thatallows only a fraction'of the pulses from single channel analyzer 24 topass through to scaler 89. This fraction is usually from between about0.8 and about 1.0 and compensates for a slight built-in imbalance in thefilters. Together with the selected time interval, this facilitatesdirect readout from the display unit 93 in milligrams per squarecentimeter of the element of interest in the specimen under study. Atthe end of the predetermined time interval, the program control unit 91turns on a gated oscillator 94 which counts the same number of pulsesinto each of the scalers 89 and 90 until scaler 89 overflows. Theoverflow pulse causes the program control 91 to cut off the gatedoscillator 94. At this time, scaler 90 will contain the differencebetween the two timed counts into scalers 89 and 90. Since the timedcount into scaler 90 will always be greater than the timed counts intoscaler 89 (except for statistical deviations), the count in scaler 90represents the concentration of the element of interest in the specimenanalyzed, and this concentration is read directly from display unit 93.Because of statistical deviations, scaler 90 will occasionally havefewer counts recorded than are H recorded in scale'r 89 prior toactuationof the gated oscillator 94. For this reason, an overflow signalfrom both sealers 89 and 90 is required on circuits 140 and 141 for azero reading to be displayed in the unit 93.

The operation of this embodiment of the apparatus of this invention maybe further explained by reference to FIGS.v 2, 3 ands of the drawings,in which the eleto their-primed counterpart elements of FIG. 1. It'canbe seen that in a-practic'a'l embodiment, rather than using singleelement-filters, the filters 19 and are typically comprised of a numberof identical elements employed in a configuration as illustrated in FIG.3. The scintillators 17 and 18 (in FIG. 2), as well as theirrespectively corresponding radiation filters 19 and 20, areencapsulatedin a radiation processing capsule 13. This radiation processing capsuleis comprised of a fluorescent radiation entrance window 43 laterallyretained by radiation processor housing 42. The radiation filters 19 and20 are positioned against the interior surface of the entrance window 43and scintillators l7 and transmitting .filters 20. The filter blanksj41do not attenuate to any appreciable degree the scintillations emanatingfrom the scintillators 18, however, andthe light transmission factor ofthe light filter blanks 411is as large as possible and approacheslOOpercent. Alight pipe or window 45 completes the structureof-theradiation processing capsule 13.

To quantitatively detect the presence of the element of interest in aspecimemsuch. asa specimen 22 in FIG. 5, the specimen is subjected to aprimary source of raments marked with unprimed designation correspond 6arm 49 and source shield 48 rotate perpendicular to the plane of FIG. 5thereby allowing a primary radiation source 11 to emit x-rays that passthrough the plastic vwindow 66 to irradiate the specimen. The primaryradiation source 11 is shielded from radiation processing capsule 13 andis'held in place at the geometric center of radiation processing unit 13by support 69 which is fastened toend closure 71.

Astrigger 61 is depressed, it actuates a microswitch 63 which sendssignals to the program control unit 91 of FIG. 1 by way of an electricalinterlock 65.

As thespecimen is irradiated, it emits fluorescent radiation, a fractionof which, along with scattered radiation, returns to the radiation probeand passes through the window 66 and support 69 to strike the radiationprocessing unit 13. Thereon it exposes simultaneously the radiationabsorbing filter 19, which may be considered to be the first filtermeans, and the radiation transmitting filter 20, which may be consideredas the sec. ond filter means. For application to the measurement of thelead content of paint, the first filter is typically comprised ofrhenium metal, while the second filter is typically comprised of iridiummetal. These particular metals are used because they have very similarattenuation characteristics for radiation over a wide range of energieswith the notable exception of the narrow energy range from about 73mabout 75 keV. In the energy band from 73 to 75 keV the iridium filter 20transmits radiation to a much greater degree than does the rheniumfilter 19. This narrow energy range embraces the dominant K x-rayemanations of lead. It would also be possible to use othermaterials; forexample, tungsten and platinum or tantalum and gold, for the filters l9and 20.

Theradiation transmitted through the filters l9 and 20 ispassed-separately 'to adistinct scintillating mediation to producefluorescent radiation therein. One I particularly useful application isthe-determination of the lead content of paint. In this instance, aspecimen of paint is exposed .to radiation from a primary radiationsource 11. Typically, a radiation source 11 is com prised of x-raysources such as cobalt-57. A paint sample is exposed to the cobalt-57radiation by grasping the handle 64 of the fluorescent radiationdetection appad forces cam 56 forward in opposition to the bias of thecompression spring 59. The upper surface of cam 56 rides in alongitudinal groove 74 in radiation probe housing 47. Cam 56 is alsoguided in its longitudinal motion by pin 57 which moves within the,guide cavity 58 in the end enclosure 71 ofthe radiation probe 10.

diumfassociated therewith to produce corresponding lightscintillations'from scintillators 17 and 18. The light'scintillationsare processed by attenuating by at 7 least about 10 percent, andpreferably by at least 50 percent, the intensity of scintillationsoccurring in the scintillating mediums associated with either theradiation absorbing filters 19 or the radiation transmitting filters 20,but not both. From the radiation processing unit 13, light from thescintillators is passed to the photosensitive surface of thephotodetector 12. Photodetector 12 generates electrical pulses havingamplitudes proportionalto the intensity of scintillations received.These electrical pulses are amplified and discrimination is carried outamong the electrical pulses by ac cepting pulses in a pulse amplituderange such as SCA 25, which pulse amplitude range includes a greatpercentage of pulses characteristic of the element of interest as wellas other pulse energies. That is, in the case of determining the leadcontentv in a specimen, the pulse amplitu de range of SCA 25 willinclude, but is not 1 limited to, pulses from the energy band between 73and As cam 56 moves forward,its'lower surface forcesf'.

the adjustable cam follower .55 downward, thereby causing an actuatorarm 53 to rotate in a counterclockwise direction about the pivot pin 54.A spline '52 attached to actuator arm- 53, rides in a verticaltr'ack(not shown) in end enclosure 71, and passes between-two adjacent teethof thespur gearS 1.v As spline, 52 and actuator arm 53 are rotatedconterclockwise in'FIG'. 5,

spur gear 51 is also turned about its axis 50,;carrying 75 keV. Whilethere is only a single radiation energy range of acceptance, the use oflight filter 21 makes 'twoelectrical pulses amplitude ranges necessary.The

electrical pulse limits of SCA 24 are equal in energy to the pulses ofthefirst pulse amplitude range diminished by the amount by which lightfilter 21 attenuates incident light energy. That is, if light emanatingfrom light v filte r 21 has an intensity of only 30 percent, of the in-,tensity of light incident upon light filter 21, the upper and lowerdiscriminators for SCA 24 should be 30 percent respectively of the upperand lower discriminators for SCA 25. The'number of electrical pulses inthe electricalpulse amplitude range SCA 25, and the product ofsubtraction is then compared with a corresponding number. associatedwith a known amount of the element of interest in order to determine theextent to which the element of interest is present in the specimen 22.In other words, the number recorded in the counter 26 is compared with anumber previously recorded inexamining a specimenv containing a knownamount of lead. The ratio between these numbers obtained from counter 26isindicative of the amount of lead present in the specimen 22.

An-alternative embodiment of a radiation detection apparatus of thisinventionis illustrated in FIG. 6. The

embodiment differs from that of FIG. 1 in several ways.

Three'scintillators, 17, l8and 118, areusedalong with two-classes oflight filters 21 and 121. As in otherembodiments, and as depicted inFIG. 3, a class will include aplurality of filter elements where aplurality of scintillator elements are'used. Each of the filter elementswithin "a class of filters is positioned between the photodetector 12and a singl escintillator, as illustrated.

' Theattenuation of-each class of light filters differs from that of anyother class by at least 10 percent. Therefore, if filter 121 attenuateslight to the greatest degree, the light reaching photodetector 12 fromscintillator 17 will be attenuated by at least 10 percent when comparedwith .the light from scintillator 18, while the light fromscintillator118- will be attenuated by at least 20- .percent and by at least 10percent more than attenuation of light'from scintillator 17. Forexample, filter 21 may attenuate lightby 40 percent while filter 121attenuates light by 70 percent. In'a corresponding fashion, the signalanalyzers 24, 25 and 1.25 are adjusted to accept electrical pulses inseparate predetermined a'mplituderanges. Signal analyzer 25 is adjustedto accept electrical pulses inan energy range within which unattenuatedelectrical pulses fall. Signal'analyzers 24 and 125 are. adjusted toaccept electrical pulses having-am! plitudes corresponding to. theenergy range SCA 25, but diminishedto theextent that the associatedlight filter attenuates incident light energy, That is, the acceptablerange of channel analyzer 125 will be lower than the acceptable range ofchannel analyzer 24 to the extent that the attenuation of filter121'exceedsthe atthat balanced radiation filters are not err ployec l.To utilize the device, radiation source of known radiationcharacteristics may be positioned proximate to only the scintillator 18.Lead radiation shields 81 and 81 are interposed between the knownradiation source 80 an'd' fl -f.

scintillators 17 and 118 so that radiation from substances 22' and 180to be analyzed do not produce scintillations in scintillator 18 and sothat radiation source 80 does not produce scintillations inscintillators I 17 and 118. In this way, the pulses counted in windowSCA 24 and in the pulse amplitude window of channel analyzer may becompared against pulses from the known source counted in window SCA 25so that a plurality of specimens may be concurrently compared against auniform reference, such as radiation source 80. Alternatively, ofcourse, the radiation source 80 could be located near either thescintillator 17 or the scintillator 118 and shielded from the otherscintillators. It is only important that source 80 not causescintillations in more than one of the scintillators and that radiationsfrom substances 22 and produce scintillations 'only in their associatedscintillator.

In an alternative use of the device of FIG. 6, three unknown radiationsources may be concurrently analyzed and compared against knownstandards. Each unknown radiation source is positioned proximate to aseparate one of the scintillators as illustrated. The lead shields 81act to shield each scintillator from radiation emanating from each ofthe radiation sources positioned proximate thereto. An optical shieldabout each scintillator is also used to isolate each scintillator fromthe other scintillators. As before, scintillations reaching thephotodetector are selectively optically attenuated. The degree ofattenuation of scintillations from each of .thescintillators 17, 18 and118 differs from the degree of attenuation of light from any otherscintillator by at least 20 percent. The pulses falling within theamplitude windows of signal analyzers 24, 25 and 125 may then beidentified as to the scintillator causing the pulse. That is,scintillations having an amplitude of interest occurring in scintillator18 will cause a pulse to emanate from channel analyzer 25, whilescintillations occurring in scintillators l7 and 118 will respectivelycause pulses to emanate from channel analyzers 24 and 125. In thismanner, a single photodetector 12 may be used for the concurrentanalysis of radioactive sources in radiation measuring devices.

The foregoing illustrative examples and drawings are provided forpurposes of explanation only, and no limitation should be construedtherefrom beyond those requirements defined by the claims of thisinvention.

We claim as our invention:

1.-In a balanced fluorescent radiation detection apparatuis utilizing aprimary radiation source, a single photodetector'having a photosensitiveface, separate scintillator means in optical communication with saidphotosensitive face and shielded from said primary radiation sourceandoptically shielded from each other, first and second radiation filtermeans wherein said first filter means, in contrast to said second filtermeans, selectively attenuates fluorescent radiation in a narrow energyband corresponding to the fluorescent radiation energy of an element ofinterest, the improvement wherein said first and second filter means areconcurrently positioned for fluorescent radiation detection inassociation with separate ones-of the scintillator means, and a lightfilter for'uniformly attenuating light energy by at least 10 percent isinterposed between the scintillator means associated with said firstfilter means and the photosensitive face of said photodetector.

2. The apparatus of claim 1 wherein said primary radiation source iscomprised of cobalt-57 and lead is the element of interest.

' 3. The apparatus of claim 2 wherein said first filter means iscomprised of rhenium metal and said second filter means is comprised ofiridium metal.

4. In a balanced filter fluorescent radiation-detection apparatusutilizing a primary radiation source, a single photodetector having aphotosensitive face, separate scintillator means in opticalcommunication with said photosensitive face and shielded from saidprimary radiation source and optically shielded from each other, firstand second radiation filter means wherein said first filter means, incontrast to said second filter means, selectively attenuates fluorescentradiation in a narrow energy band corresponding to the fluorescentradiation peak of an element of interest, the improvement wherein saidfirst and second filter means are concurrently positioned forfluorescent radiation detection in associated with separate ones of thescintillator means, and a light filter for uniformly attenuating lightenergy by at least percent is interposed between the scintillator meansassociated with said second filter means and the photosensitive face ofsaid photodetector.

5. A fluorescent radiation detection apparatus comprising:

a. a primary radiation source,

b. a single photodetector having a photosensitive surface,

c. distinct scintillator means optically shielded from each other and inoptical communication with said photosensitive face,

d. a radiation shield separating said primary radiation source from eachof said scintillator means,

e. a radiation absorbing filter that is particularly responsive inabsorbing radiation of a predetermined narrow energy range correspondingto the fluorescent radiation energy of an element of interest, locatedproximate to one of the aforesaid scintillator means, and

f. a radiation transmitting filter having energy response attenuationcharacteristics similar to those of said radiation absorbing filter atenergy levels other than in the aforesaid predetermined range locatedproximate to another of the aforesaid scintillator means,

. a light filter for uniformly attenuating incident light energy by atleast 10 percent positioned between said photosensitive surface and thescintillator means associated with said radiation absorbing filter,

h. separate channel analyzers connected in parallel to saidphotodetector wherein the electrical pulse energy window of one of saidchannel analyzers is set to accept those electrical pulses havingamplitudes corresponding to the energy window of the other of saidchannel analyzers but diminished by the amount by which the aforesaidlight filter attenuates incident light energy, and

i. counting means connected to said channel analyzers.

6. A fluorescent radiation detection apparatus comprising:

a. a primary radiation source,

b. a single photodetector having a photosensitive surface,

0. distinct scintillator means optically shielded from each other and inoptical communication with said photosensitive surface,

d. a radiation shield separating said primary radiation source from eachof said scintillator means,

e. a radiation absorbing filter that is particularly responsive inabsorbing radiation of a predetermined narrow energy range correspondingto the fluorescent radiation peak of an element of interest locatedproximate to one of the aforesaid scintillator means,

f. a radiation transmitting filter having energy response attenuationcharacteristics similar to those of said radiation absorbing filter atsome energy levels other than in the aforesaid predetermined rangelocated proximate to another of the aforesaid scintillator means,

. a light filter for uniformly attenuating incident light energy by atleast 10 percent positioned between said photosensitive surface and thescintillator means associated with said radiation transmitting filter,

h. separate channel analyzers connected in parallel to saidphotodetector wherein the electrical pulse energy window of one of saidchannel analyzers is set to accept those electrical pulses havingamplitudes falling within the energy window of the other of said channelanalyzers, but diminished by the amount by which the aforesaid lightfilter attenuates incident light energy, and

i. counting means connected to said channel analyzers.

7. A method of quantitatively detecting the presence of an element ofinterest in a specimen comprising subjecting the specimen to a primarysource of radiation to produce fluorescent radiation therein, exposing aradiation absorbing filter which selectively attenuates radiation of apredetermined narrow energy range corresponding to the fluorescentradiation peak of an element of interest to said fluorescent radiation,exposing a radiation transmitting filter having energy attenuationcharacteristics similar to those of said radiation absorbing filter atsome energy levels other than in the aforesaid predetermined energyrange to said fluorescent radiation, separately passing fluorescentradiation transmitted through each of the aforesaid radiation filters toa distinct scintillating medium to produce corresponding lightscintillations, uniformly processing said light scintillations byoptically attenuating by at least 10 percent the intensity ofscintillations occurring in the scintillating medium associated withonly one of said radiation absorbing and radiation transmitting filters,passing said light scintillations from all of said scintillators to asingle photodetector, generating electrical pulses having amplitudesproportional to the light intensity of scintillations, discriminatingagainst said electrical pulses by accepting pulses in a first electricalpulse amplitude range including a pulse ampitude peak of the aforesaidelement of interest and also other pulse energies and by accepting thosepulses in a second electrical pulse amplitude range which correspond inenergy to pulse amplitudes falling within said first pulse amplituderange diminished to the extent that optical attenuation ofscintillations occurs in those scintillations which are opticallyattenuated, subtracting a fixed portion of the number of pulses in oneelectrical pulse amplitude range from the number of pulses in the otherelectrical pulse amplitude range, and comparing the product ofsubtraction with a corresponding number for a known amount of saidelement of interest to determine the extent to which the aforesaidelement of interest is present in the aforesaid specimen.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No.3,859,525 Dated January 7, 1975 Invent0r(s) John Bo Ashe; Peter F.Berry; James D. Hall It is certified that error appears in theabove-identified patent and that said Letters Patent are herebycorrected as shown below:

Signed and Sea-led this twenty-fifth Of November 1975 {SEAL} Arrest:

R'UTH C. MASON C. MARSHALL DANN .-l.tresting Officer Commissioneru'j'Patents and Trademarks

